The invention concerns the field of high flow spectrometry using x-rays and gamma rays.
The applications of high flow spectrometry are varied.
Applications include the use of gamma probes in radiation protection, multi-energy imaging in the medical field (e.g. bi-energy scanners), in the field of non-destructive testing and in security applications (e.g. detection of explosive materials using multi-energy radiography).
One particular industrial application of the invention is the detection of explosives for examining baggage using moving radiographs. But other applications are possible, in particular during measurements of intense X and/or gamma photonic flows using spectrometry, for example in measuring waste or nuclear fuels.
Moreover, it is difficult to make the known techniques compatible with the current baggage inspection requirements: a method is needed that is fast, but also precise and compatible with security. In particular, the movement speed of the baggage requires that a measurement be done of the energy of the photons transmitted through the baggage, over a short time (several ms) with a high incident photon flow (several tens of Mphotons/mm2/s) to keep sufficient statistics.
The spectrometric sensors concerned by the invention are preferably direct conversion sensors, i.e. the incident X photons on the sensor interact with a polarized semiconductor material (CdTe for example), and create a cloud of electronic charges (typically 10000 electrons for an X photon of 60 keV).
These charges are then collected by electrodes and form a transient electrical signal called a pulse. If the collection is complete, the entire measured pulse is proportionate to the energy deposited by the incident particle.
An electronic circuit makes it possible to measure this integral.
After digitization, the pulses are classified in different channels depending on their amplitude, and an energy value is assigned to each channel. The distribution by channels of each interaction corresponds to the energy spectrum of the radiation having interacted with the irradiated object, or energy spectrum of the detected radiation. The radiation is preferably an X or gamma photon radiation.
In the case of baggage inspection, such a spectrum makes it possible to provide information on the density and nature of said object.
In spectrometry systems, comprising a sensor connected to electronic circuits for amplifying and processing the detected signal, the problem arises of correcting degradation effects related to the high flows of photons on the measured spectrums (as indicated above, high flows are indeed necessary during baggage inspections).
More specifically, it involves the poor separability, or stacking, phenomenon of interactions detected by the detector at very close moments. The more intense the incident photon flow at the detector, the higher the interaction rate (number of interactions occurring in the detector per unit of time). First, the counting rate measured by the detector, corresponding to the number of interactions detected per unit of time, increases with the interaction rate, and the probability of obtaining a stack also increases. Then, when the incident photon flow becomes too significant, the counting rate practically does not increase any more, and can even decrease, due to the saturation of the detector.
The notion of strong flow corresponds to typical flow values between 1×104 and 109 interactions per second and per pixel (or elementary detector). In the case of X ray with energy in the vicinity of 100 keV, the number of interactions per second is relatively close to the number of incident photons per second, in other words the incident photon flow, the probability of interaction of such photons being high.
One important parameter is the counting rate measured by the detector previously defined. When the flow of photons to which the detector is subjected is not too high, the interaction rate in the detector is substantially equal to the counting rate measured by the detector, the latter corresponding to the number of events (or number of counts) appearing in the spectrum per unit of time.
In the case of an intense flow of radiation, in general, beyond a given counting rate, a saturation of the detector and the signal processing electronics occurs.
The measured counting rate then no longer corresponds to the flow to which the detector is subjected.
One consequence of this saturation is a strong degradation of the energy resolution of the spectrum and the detection efficacy.
FIG. 9 is a spectrum measurement at two different flows, which illustrates the problems posed by the stacking phenomenon.
Curve I corresponds to an incident flow of 6,082×106 photons/s/detector while curve II corresponds to an incident flow of 4,752×104 photons/s/detector.
When the flow increases (number of incident X photons per unit of time per pixel), the supplied signal deteriorates due to the stacking phenomenon: if two events are detected with too short a time lapse separating them, the system is not able to tell them apart and provides an erroneous signal depending on the energies of the two photons and the time interval separating them.
In FIG. 9, two effects resulting from the stacking phenomenon can be seen on curves I and II:                a decrease in the counting rate measured when the flow increases, visible at low energies (zone A in FIG. 9);        an increase in the number of events counted at high energies with the flow due to the spectrum of stacks (zone B in FIG. 9).        
This stacking phenomenon is well known. Different method classes exist making it possible to deal with the stacking phenomenon.
Empirical methods are known: one approach is based on calibrating the stacking phenomenon with radioactive sources having known activity.
The information resulting from the calibration is then used on the unknown signal, as described in American National Standard for Calibration and Use of Germanium Spectrometers for the Measurement of Gamma-Ray Emission Rates of Radionuclides, American National Standards Institute (ANSI) N42.14-1999, p. 7, 13, 15, 86, 89, 134.
The main drawback of this approach is the need to have γ-ray emitting sources with strong activity, which makes the calibration method complex and in particular poses radiation protection problems.
Also known are analog methods, which optimize the electronics to minimize the stacks. In particular the use of inhibitor circuits makes it possible not to take into account the new particles absorbed before the end of the processing of the current particle.
This type of approach makes it possible to obtain a non-paralyzable system, the drawback being that the dead time resulting from the processing decreases the counting rate performance of such a system.
Digital methods also exist, called live time correction methods, that make it possible to reject part of the stacks. But this method causes an increase in the time to acquire a spectrum.
According to other solutions, certain signal processing parameters are adjusted, in particular concerning the shaping of the impulses. But, aside from degradation of the resolution, these solutions are not very efficient: they only slightly push back the boundary of the interaction rate from which the measuring system is no longer exploitable.
Lastly, there are a posteriori correction methods, in particular that from document FR 2 870 603, or described in Trigano, T., Traitement du signal spectrométrique: <<Etude du désempilement de spectre en énergie pour la spectrométrie gamma>>, 2006.
This method is based on knowledge of the duration and energy of each pulse, which is a limitation on said method, in particular at high counting rates.